It is proposed to determine the on and off rate constants for dimer exchange with microtubules reassembled to steady state from outer doublet and bovine brain tubulins. The effect of colchicine-tubulin dimer complexes (CDs), as well as chemically unique tubulin populations derived from A- and B-subfibre microtubules, on the rate constants will be examined. CDs are conformationally different from drug-free dimers, and there is evidence indicating that chemical modification of tubulin also induces conformational changes in the tubulin dimer. These structural changes in the tubulin dimer potentially provide a basis by which microtubule properties and functional specificity for accessory protein binding may be determined. If tubulin conformation is relevant for microtubule properties and functions, an analysis of CDs and chemically distinct tubulin populations on microtubule association-dissociation kinetics should reveal kinetic changes of possible physiological significance. In addition, it may be possible to assay changes in microtubule surface lattice produced by CDs or tubulins by examining the reassociation kinetics of axonemal proteins (eg. dynein) with the reconstituted microtubules.